An image-forming apparatus such as a copying machine and a printer which uses an electrophotographic system is provided with a developing device for developing an electrostatic latent image formed on the surface of a photoconductive member.
In recent years, in response to demands for miniaturization of image-forming apparatuses, there have been ever-increasing demands for miniaturization of developing devices, with the result that techniques for ensuring the developing characteristics as well as for achieving miniaturization has been demanded.
For example, a developing device has been put into practical use in which a developing roller using the magnetic brush system, which carries a two-component developer consisting of toner and magnetic carrier to a developing area facing a photoconductive member by utilizing a magnetic force, is provided, and after completion of the developing process, the developer is recovered into a developing vessel.
In the above-mentioned magnetic brush system, in order to stabilize the developing process, it is necessary to supply toner to be consumed and also to control the ratio of the toner contained in the developer, that is, the toner density, to be constant.
Normally, in the developing device using the above-mentioned magnetic brush developing system, the carrier accounts for a greater ratio in the developer, making it necessary to increase the size of the developer vessel storing the developer; this tends to make an entire developing device larger. Moreover, stirring members, etc., are required so as to control the toner density and to make constant the quantity of charge in the toner in the developer, and this plurality of stirring members are one of the reasons that miniaturization of the developing device has been limited.
Recently, in order to avoid the above problem, a developing device of one-component developer system, which uses a one-component developer consisting of toner without containing a carrier, has been proposed and put into practical use. In the developing device using such a one-component developer, it is neither necessary to control the toner density, nor to provide a carrier, so that the volume of the developing vessel can be greatly reduced, thereby making it possible to miniaturize the developing device.
Moreover, this one-component developer system is superior in ease of maintenance, etc. In other words, since no exchange for deteriorated developer, especially for developer containing deteriorated carrier, is required, no corresponding maintenance is required.
Furthermore, since it is only necessary to replenish toner, and since neither detection of the toner density nor control for the detection is required, this system makes it possible to simplify the controlling process. In other words, the developing device using one-component toner only requires refilling of toner on demand.
In the developing device having the above-mentioned arrangement, predetermined voltages are applied to respective members constituting the developing device, such as the developing roller and supplying roller, in order to develop an electrostatic latent image formed on the photoconductive member with high fidelity.
Therefore, electrical characteristics of toner and the respective members form major factors in determining the developing characteristic. In particular, the quantity of charge in toner and the resistivity of the developing roller greatly affect the developing characteristic.
The relationship between the specific charge q/m (quantity of charge per unit mass) and the developing characteristic is indicated by a characteristic shown in FIG. 10, wherein, when the specific charge is small, the electric potential difference from the developing start voltage to the developing end voltage is small. Hereinafter, this characteristic is referred to as "a rising developing gamma characteristic". In contrast, when the specific charge is large, the electric potential difference from the developing start voltage to the developing end voltage is large. Hereinafter, this characteristic is referred to as "a flat developing gamma characteristic". Therefore, if a developing device has great fluctuations in the toner specific charge, it has great fluctuations in the developing characteristics, failing to form good images. Since conventional one-component developing devices fail to sufficiently suppress the fluctuations in the specific charge, they tend to have degradation in the image quality, for example, the occurrence of developing ghost images.
Moreover, the relationship between the developing roller resistivity and the developing characteristics is indicated by a characteristic shown in FIG. 8, wherein the greater the resistivity, the more flat developing gamma characteristic is exerted. For example, in the case of application of a developing roller having a high resistivity which has been conventionally proposed, since the developing roller is highly susceptible to resistivity fluctuations under the influence of temperatures and humidities, the developing characteristics change greatly, resulting in a problem of, for example, great fluctuations in the image density. Moreover, this also causes degradation in the image quality, such as the occurrence of developing ghost images, due to a charge accumulated on the surface of the highly resistive layer of the developing roller.
Therefore, in the developing device using a one-component developer, it has been proposed that the toner charging characteristic be stabilized by using a charge-control agent and an externally added agent and that the developing characteristic be stabilized by allowing the developing roller to have a low resistivity; however, these efforts have not sufficiently achieved high image-quality yet.
More specifically, with respect to the specific charge fluctuations in the one-component developer, the developer inherently contains specific charge fluctuations due to the distribution of toner particle sizes, that is, the specific charge fluctuates simultaneously with particle size fluctuations.
Moreover, with respect to the developing roller resistivity, the application of a highly resistive developing roller tends to cause a problem of the occurrence of developing ghost images due to a charge accumulated on the surface of the highly resistive layer; in contrast, the application of a low resistive developing roller tends to cause a problem of degradation in the image quality due to dielectric breakdown in the toner layer and over-currents.